134 



THE GARDENER'S ASSISTANT. 



of the various plots in an experimental wheat- 

 field at Rothamsted were sampled in October, 

 1881, to the depth of 27 inches. There had 

 been much rain after harvest, and the condi- 

 tions were very favourable to nitrification. The 

 nitrates were found to be located chiefly near 

 the surface, the distribution in the three depths, 

 each of 9 inches, being on the average: 1st 

 9 inches, 100 lbs. of nitrogen per acre; 2nd 

 9 inches, 59 lbs. of nitrogen per acre; 3rd 

 9 inches, 31 lbs. of nitrogen per acre. The 

 quantity of nitrates found in these soils gener- 

 ally bore a distinct relation to the amount of 

 the preceding crop. It appears that the organic 

 nitrogen of permanently unmanured or very 

 poor land nitrifies with more difficulty than 

 does the nitrogen of land that has yielded large 

 crops, or which has received farmyard or other 

 organic manures. It is very important, there- 

 fore, for gardeners to bear in mind that the 

 nitrogenous capital of a soil, which represents 

 to a considerable extent its fertility and power 

 of yielding remunerative crops, depends, as a 

 rule, on the bulk and composition of the previous 

 plant residues. The present condition of a soil is 

 thus in great measure a consequence of its past 

 fertility. 



In ordinary soils the four constituents of 

 sand, lime, clay, and organic matter are gener- 

 ally to be found. In the special soils used in 

 horticulture it is usually the organic matter 

 that predominates and gives to the mould its 

 characteristic properties. 



In regard to potash and phosphoric acid, 

 which are present in all good soils, it may be 

 assumed that they are dissolved by the acid 

 secretions from the plants where the roots are 

 in direct contact with these elements. 



Humus matter may be taken up in a similar 

 manner, and by some plants more readily than 

 by others. Still, it is not until the nitrogen of 

 the humus has assumed the form of nitric acid 

 that its full eft'ect upon vegetation is realized. 



Ferments of the Soil — We now know that the 

 production of nitrates in the soil — a process of 

 the greatest importance for the nutrition of all 

 horticultural crops — is accomplished by the 

 action of two organisms (bacteria), each of 

 which performs a distinct stage in the work. 

 Warington informs us that by one organism 

 ammonium carbonate is oxidized, and the nitro- 

 gen converted into a nitrite. By the second 

 organism nitrites are converted into nitrates. We 

 have here an excellent example of the way in 

 which certain special functions — that is, certain 

 narrowly -limited lines of work — are exercised 



by individual species of bacteria. The nitrous 

 organism can oxidize ammonia to nitrite, but it 

 cannot change a nitrite into a nitrate. The 

 nitric organism, on the other hand, oxidizes 

 nitrites readily, but it cannot oxidize ammonia. 

 Both of these organisms are present in all fertile 

 soils, but the formation of nitrites is not usually 

 perceived, as they are at once converted into 

 nitrates. 



It may be taken as a general rule that in the 

 absence of the element oxygen, one of the con- 

 stituents of atmospheric air, bacteria act as a 

 ferment in the soil, splitting or breaking up the 

 organic matter present into the new compounds 

 mentioned above, while in the presence of oxy- 

 gen the bacteria become the active oxidizing 

 agents. The most important ferment of the soil 

 is decidedly the nitric ferment, that abounds, as 

 we have said, in all fertile soils, and sets free 

 the nitrogen locked up in the otherwise inert 

 organic combinations, and so offers it under a 

 soluble and assimilable form to the plants. 



One essential condition for the active processes 

 of oxidation and decomposition is, of course, the 

 presence of air; an open porous soil is thus far 

 more exposed to oxidation and nitrification than 

 one in a closely consolidated condition; hence 

 arises the beneficial effect of mixing porous sub- 

 stances, such as peat, charcoal, and sand, with 

 stiff horticultural moulds. The operations of 

 tillage also tend to promote in the soil oxida- 

 tion of the organic matter, and assist in its 

 nitrification. 



A sufficiency of water is essential for the 

 activity of all living agents; oxidation and 

 decay are thus far more rapid in a moist soil 

 than in a dry one. The constant waterings 

 given to plants in a well-conducted garden 

 provide this condition. It must, however, be 

 remembered that a great excess of water is 

 fatal to oxidation, the admission of air being 

 excluded as soon as the soil is filled with water. 



Temperature is another prime factor in deter- 

 mining the rate of oxidation and nitrification in 

 mould ; the activity of all living agents, whether 

 animal or vegetable, being dependent on the 

 occurrence of a favourable degree of heat, and 

 being confined to certain specific ranges of tem- 

 perature. Oxidation is consequently found to 

 be far more rapid in summer than in winter, 

 and much more energetic in hot climates than 

 in cold; accordingly we find it more active in 

 a conservatory than in the open garden. 



It is known, further, that the nitrifying organ- 

 ism cannot carry on its work unless it is furnished 

 with some alkaline substance to neutralize the 



